Networking effect
Mastodon required people to join a server which made out to people who are used to signing up to a single website for the past 20 years or such, seam too complex.
You also end up in a situation where social media managers have to reach engagement and impression targets and don't think which is best platform that will not be taken over Elon musk or Mark Zuckerberg style, but which will reach engagement targets the quickest.
A micky take out of enterprise ending?